In recent years, noncontact communication technology for interchanging signals by electromagnetic induction has been established and increasingly used for transportation tickets and electronic money. Also, this noncontact communication function tends to be mounted also on a portable phone, and this trend is expected to be developed more in the future. Not only in proximity communication by electromagnetic induction but also in logistics, an IC tag readable or writable at a distance of several meters has also been commercialized. Furthermore, this noncontact communication technology allows not only noncontact communications but also power transmission at the same time, and therefore can also be implemented on an IC card without a power supply such as a battery in itself.
As antenna modules for RFID (Radio Frequency Identification) to which this noncontact communication technology is applied, several types have been conventionally used as follows. Firstly, there is an antenna module having a coil pattern fabricated on a flat surface by using a FPC (Flexible Printed Circuit) or a rigid substrate. Secondly, there is an antenna module having a coil fabricated by winding a circular cross-sectional wire. Thirdly, there is an antenna module having a coil formed by taking a FPC, a FFC (Flexible Flat Cable), or the like as a harness and shaping that harness into a ring.
Any of the antenna modules described above is selected as appropriate according to the design in consideration of the arrangement and shapes of components, and is incorporated in an electronic device for use.
When an antenna module is arranged in an electronic device, due to an influence of a metal used in a metal-made casing or an internal component of the electronic device, magnetic flux oscillated from a reader/writer cannot be efficiently drawn to the antenna coil. To avoid such suffering from this metal influence, a ferrite-made magnetic sheet with a relatively high-permeability and a small loss factor is mounted on the perimeter of the antenna in the antenna module.
For example, FIG. 12 depicts, sequentially from left, an inductance of a single antenna coil, an inductance of an antenna coil in proximity to a metal body, and an inductance of an antenna coil when a magnetic sheet is arranged between the antenna coil and the metal body.
As such, with a ferrite-made magnetic sheet having an excellent magnetic characteristic being arranged so as to be superposed on the antenna module, a magnetic field is prevented from entering the metal arranged around the antenna module to become an eddy current to be changed into heat. Also, the ferrite-made magnetic sheet has its shape, combination, and others optimized so as to obtain excellent communication performance. Furthermore, to make a portable electronic device such as a portable phone thinner, the antenna module is desired to be made as thin as possible, as being laminated with the ferrite-made magnetic sheet.
Still further, in a communication system to which this noncontact communication is applied, a resonant capacitor is connected to a loop antenna for performing noncontact communications and power transmission between a reader/writer and a noncontact data carrier, and a resonance frequency represented by f=1/(2π(LC)1/2) is matched with a normal frequency of the system, thereby allowing stable communications between the reader/writer and the noncontact data carrier and maximizing a communication distance. L and C determined by the characteristics of the loop antenna and the resonant capacitor have several variable factors, and each do not necessarily have an assumed value. For example, in a communication system with a normal frequency of 13.56 [MHz] and for the use purpose of transportation tickets and electronic money, in view of reliability, the resonance frequency of a resonant circuit of an antenna module is required to be suppressed to be on the order of 13.56 [MHz]±200 [KHz] even if the system receives an influence of the variable factors described above.
Here, in the noncontact data carrier, the loop antenna is formed of a copper foil pattern to decrease cost, and the value of L is varied due to a deviation of the pattern width or the like. Regarding a temperature change rate of each of C determined by the characteristics of a general chip capacitor and L determined by the characteristics of the antenna coil, a variation of L with respect to C may be on the order of 100-fold in level. For example, if the value of L is 2.5 [μH] and 1% is displaced, the resonance frequency is shifted by 70 KHz, and therefore minimal fluctuations with respect to the temperature of the L value are desired.
Patent Document 1 describes a communication device in order to prevent the resonance frequency from fluctuating due to temperature changes as described above, the communication device including a temperature detecting unit and a frequency shift that shifts the resonance frequency to be tuned at a tuning unit according to a temperature detected from that temperature detection.